The presence of a slide valve in a duct in a vacuum installation requires a second valve generally referred to as a "bypass" valve to be connected in parallel therewith.
Thus, one known configuration comprises a slide valve coupled in parallel with a second valve of the "gas valve" type which makes it possible to reestablished pressure equilibrium on opposite sides of the valve member of the slide valve prior to opening it.
The "gas valve" thus serves to avoid excessive molecular disturbance between the two previously-isolated portions of the duct when opening the slide valve.
This system requiring two valves is expensive and also requires a special configuration to enable the gas valve to be integrated in parallel as a bypass valve.